Most film versions of Shakespeare's plays have to leave something out...either due to time-constraints or because film directors and screenplay writers are afraid modern audiences aren't going to understand archaic references in his works.
One example from the Branagh "Much Ado" that is significantly different from the play is the scene in which Don John actually takes Claudio and Don Pedro to look up at Hero's window. They are seeing Borachio "wooing" Margaret, but Don John has told them that that is really Hero. In the play, that scene doesn't exist. We go from Don John telling Don Pedro and Claudio about Hero's unfaithfulness the night before the wedding to the actual wedding day itself where Claudio disgraces Hero in front of everyone. This scene added by Branagh would have fallen somewhere between Act 3, scene 2 and Act 4, scene 1.
Another example is a conversation between Benedick and Margaret in Act 5, scene 2. In the film, we see Benedick trying to write a sonnet to Beatrice, but he says that he "was not born under a rhyming planet." But if you read the play, you'll see that Margaret is actually in that scene and has rather a flirtatious conversation with Benedick, to which he responds jokingly and in fun. She then leaves to go find Beatrice and bring her to Benedick.
Pop a bowl of popcorn, sit back and watch this great film, but keep your book open and read along while you watch. You'll find even more changes as you read along. :)